1. Field of the Invention
The present invention relates to a method of executing a sliding-mode control including a process of feeding back an amount of twist of a robot, and more particularly to a method of executing a sliding-mode control including a process of feeding back an amount of twist, the method being capable of reducing vibration by changing switching surfaces with a value which is produced by filtering the amount of twist.
2. Description of the Related Art
Some mechanical systems have mechanical actuators, such as robot arms, which allow the inertia of applied loads to vary greatly depending on whether the robot arms, for example, are extended or contracted, and which include servomotors and associated mechanisms that have large speed reduction ratios. If those mechanical systems are controlled by the ordinary feedback control process, their responses are so slow that they are not practically feasible.
In view of the above drawback, it has been customary to measure the spring constant and viscosity term of a mechanical actuator, such as a robot arm, in various postures thereof, and calculate a dynamically variable feed-forward term according to the conventional linear control method on an on-line basis for feed-forward control of the mechanical system. The applicant has filed Japanese Patent Application No. 1-117518 on a sliding-mode control method in which a torque command value is varied depending on the fluctuation of the inertia of a robot. As well known in the art, sliding mode control which has been used in relay control since 1955, has been applied to many kinds of systems, including robot manipulators. The use of sliding mode control in a robot control system suppresses nonlinear interactions between servo systems due to Coriolis and centrifugal forces and automatically compensates for variations in mass at the hand or distal end of the robot manipulator.
The applicant has also filed Japanese Patent Application No. 1-253767, corresponding to U.S. patent application Ser. No. 07/687,902, on a sliding-mode control method which is an improvement over the above sliding-mode control method. According to the improved sliding-mode control method, a switching plane for sliding-mode control is varied depending on a twist or positional difference between a servomotor and a mechanical actuator which is driven by the servomotor, and a twisting speed, i.e., the speed at which such a twist occurs. As illustrated in FIGS. 3A and 3B, a manipulator may have an angle .theta..sub.t between lines L.sub.1 and L.sub.2 which respectively connect points J.sub.1 and J.sub.2, and joint J.sub.2 to the actual position of end E of the manipulator. Due to twist, the angular position .theta. of the motor at joint J.sub.2 may be different than .theta..sub.t. As illustrated in FIG. 3B, the angular position .theta. of the motor may also differ from the commanded position .theta..sub.r.
The above method is however disadvantageous in that it is necessary to establish a positional loop gain lower than a resonant frequency, and the servo rigidity is lowered.